The purpose of a vehicular transmission is to provide a neutral, at least one reverse and one or more forward driving ranges that impart power from an engine, and/or other power sources, to the drive members which deliver the tractive effort from the vehicle to the terrain over which the vehicle is being driven. As such, the drive members may be front wheels, rear wheels or a track, as required to provide the desired performance.
A series propulsion system is a system in which energy follows a path from an engine to an electric storage device and then to an electrical motor which applies power to rotate the drive members. There is no direct mechanical connection between the engine and the drive members in a series propulsion system.
Transmissions adapted to receive the output power from either an engine or an electric motor, or both, have heretofore relied largely on what has been designated as series, hybrid propulsion systems. Such systems are designed with auxiliary power units (APUs) of relatively low power for minimum emissions and best fuel economy. However, such combinations of small APUs and even large energy storage devices do not accommodate high-average power vehicles or address duty cycles that demand continuous, constant speed operation. Steep grades and sustained high-average cruising speeds at desired high efficiencies are not achievable with a typical, series, hybrid transmission configuration.
The challenge, therefore, is to provide a power system that will operate at high efficiencies over a wide variety of operating conditions. Desirable electric variable transmissions should leverage the benefits of a series, hybrid transmission for desirable low-average power duty cycles—i.e., low speed start/stop duty cycles—as well as the benefits of a parallel hybrid transmission for high-average output power, high speed duty cycles. In a parallel arrangement the power supplied by the engine and the power supplied by the source of electrical energy are independently connected to the drive members.
Moreover, perfecting a concept wherein two modes, or two integrated power split gear trains, with either mode available for synchronous selection by the on-board computer to transmit power from the engine and/or the motor/generator to the output shaft results in a hybrid transmission having an extremely wide range of applications.
The desired beneficial results may be accomplished by the use of a variable, two-mode, input and compound split, parallel hybrid electro-mechanical transmission. Such a transmission utilizes an input member to receive power from the vehicle engine and a power output member to deliver power to drive the vehicle. First and second motor/generator power controllers are connected to an energy storage device, such as a batter pack, so that the energy storage devices can accept power from, and supply power to, the first and second motor/generators. A control unit regulates power flow among the energy storage devices and the motor/generators as well as between the first and second motor/generators.
A variable, two-mode, input-split, parallel, hybrid electro-mechanical transmission also employs at least one planetary gear set. The planetary gear set has an inner gear member and an outer gear member, each of which meshingly engages a plurality of planet gear members. The input member is operatively connected to one of the gear members in the planetary gear set, and means are provided operatively to connect the power output member to another of the gear members in the planetary gear set. One of the motor/generators is connected to the remaining gear member in the planetary gear set, and means are provided operatively to connect the other motor/generator to the output shaft.
Operation in the first or second mode may be selectively achieved by using torque transfer devices. Heretofore, in one mode the output speed of the transmission is generally proportional to the speed of one motor/generator, and in the second mode the output speed of the transmission is generally proportional to the speed of the other motor/generator.
In some embodiments of the variable, two-mode, input-split, parallel, hybrid electro-mechanical transmission, a second planetary gear set is employed. In addition, some embodiments may utilize three torque transfer devices—two to select the operational mode desired of the transmission and the third selectively to disconnect the transmission from the engine. In other embodiments, all three torque transfers may be utilized to select the desired operational mode of the transmission.
With reference, again, to a simple planetary gear set, the planet gear members are normally supported for rotation on a carrier that is itself rotatable. When the sun gear is held stationary and power is applied to the ring gear, the planet gear members rotate in response to the power applied to the ring gear and thus “walk” circumferentially about the fixed sun gear to effect rotation of the carrier in the same direction as the direction in which the ring gear is being rotated.
When any two members of a simple planetary gear set rotate in the same direction and at the same speed, the third member is forced to turn at the same speed, and in the same direction. For example, when the sun gear and the ring gear rotate in the same direction, and at the same speed, the planet gears do not rotate about their own axes but rather act as wedges to lock the entire unit together to effect what is known as direct drive. That is, the carrier rotates with the sun and ring gears.
However, when the two gear members rotate in the same direction, but at different speeds, the direction in which the third gear member rotates may often be determined simply by visual analysis, but in many situations the direction will not be obvious and can only be determined by knowing the number of teeth present in the gear members of the planetary gear set.
Whenever the carrier is restrained from spinning freely, and power is applied to either the sun gear or the ring gear, the planet gear members act as idlers. In that way, the driven member is rotated in the opposite direction as the drive member. Thus, in many transmission arrangements when the reverse drive range is selected, a torque transfer device serving as a brake is actuated frictionally to engage the carrier and thereby restrain it against rotation so that power applied to the sun gear will turn the ring gear in the opposite direction. Thus, if the ring gear is operatively connected to the drive wheels of a vehicle, such an arrangement is capable of reversing the rotational direction of the drive wheels, and thereby reversing the direction of the vehicle itself.
As those skilled in the art will appreciate, a transmission system using a power split arrangement will receive power from two sources. Utilization of one or more planetary gear sets permits two or more gear trains, or modes, by which to deliver power from the input member of the transmission to the output member thereof.
U.S. Pat. No. 5,558,589, which issued on Sep. 24, 1996 to General Motors Corporation and is hereby incorporated by reference, teaches a variable, two-mode, input-split, parallel, hybrid electro-mechanical transmission wherein a “mechanical point” exists in the first mode and two mechanical points exist in the second mode. U.S. Pat. No. 5,931,757, which issued on Aug. 3, 1999 to General Motors Corporation and is hereby incorporated by reference, teaches a two-mode, compound-split, electro-mechanical transmission with one mechanical point in the first mode and two mechanical points in the second mode.
A mechanical point occurs when either of the motor/generators is stationary at any time during operation of the transmission in either the first or second mode. The lack of a mechanical point is a drawback inasmuch as the maximum mechanical efficiency in the transfer of power from the engine to the output occurs when one of the motor/generators is at a mechanical point, i.e., stationary. In variable, two-mode, input-split, parallel, hybrid electro-mechanical transmissions, however, there is typically one point in the second mode at which one of the motor/generators is not rotating such that all the engine power is transferred mechanically to the output.
The two-mode, compound-split, electromechanical transmission referenced above is an efficient option for commercial-duty vehicles, such as transient buses and the like, which regularly operate at close to their maximum capacity. When the transmission is operating in electric mode and the engine is not running, the motor/generator driving the vehicle must also rotate the engine, which results in parasitic losses.